Semiconductor components which are controllable by means of the field effect, for instance power MOSFETs (metal oxide field effect transistors), are currently produced in large numbers in industrial mass production and standardly used in technical applications. Such components have a semiconductor body with first and second doped terminal zones and a channel zone between the terminal zones, which is doped complementarily to the terminal zones. A control electrode is furthermore formed so that it is insulated from the semiconductor body, and extends between the terminal zones in the vicinity of the channel zone. In a MOSFET, the first and second terminal zones form the drain and source zones of the transistor. They are formed on opposite surfaces of the semiconductor body for a vertical structure of the semiconductor component, while in a lateral structure they are arranged on the same surface of the semiconductor body.
A particular requirement for high-voltage switches (power MOSFETs) is on the one hand to achieve a good blocking ability, while on the other hand good transmission properties are desirable. Without special precautions, however, a high blocking voltage implies weak doping and a comparatively large thickness of the voltage-absorbing layer, while good transmission properties require heavy doping and a comparatively small thickness of the active layer.
In order to accommodate especially the requirement for a high blocking voltage, a more weakly doped drift zone (“epi layer”) of the same conduction type as the terminal zones, which can increase the breakdown strength, is often provided between the heavily doped first terminal zone (drain zone) and the channel zone in vertically constructed power MOSFETs, although impaired transmission properties of the switch due to the reduced charge carrier concentration must also be taken into account.
A further improvement in this regard has been provided by the development of novel high-voltage switches with a charge compensation structure, as are marketed by the Infineon Technologies AG for example under the name “CoolMOS”. In this case, the two charge carrier types in the component are spatially separated from one another in the epi layer so that the net charge balances out to approximately zero in the blocking case, while unreduced (heavy) doping of one of the two charge carrier types is available for the current flow in the on state of the transistor. This novel transistor structure with charge compensation allows a drastic reduction of the on-state resistance as well as a high blocking voltage, since the two conflicting properties of blocking ability and on-state resistance are functionally decoupled from each other.
In vertical power MOSFETs with or without charge compensation, it is conventional to short circuit the terminal zone (source zone) and the channel zone. The reason for this is the fact that with non-short circuited source and channel zones, charge carriers can accumulate in the channel zone during operation and can activate the parasitic bipolar transistor of the MOSFET, which is always present owing to the sequence of differently doped zones, the consequence of which is that the voltage strength of the component can be impaired. Since the short circuited source and channel zones are at the same potential, charge carriers cannot accumulate in the channel zone so that the parasitic bipolar transistor cannot be activated under normal operating conditions.
Short circuiting the source and channel zones, however, creates an (inverse) diode (comprising the channel zone and the drift or second terminal zone), which can block only in the forward direction (usually the drain-source current direction) of the transistor, while it conducts in the reverse direction (usually the source-drain current direction). This diode can be utilized, and acts as a freewheel diode in many power electronic applications.
If the MOSFET is to be optimized in respect of its properties as the best possible power switch, then the properties of the inverse diode can conflict with this. A limitation of the switching frequency arises, for example, the cause of which usually involves chopping of the reverse current of the inverse diode. Such chopping of the reverse current is associated with high current rates of change (di/dt) and can particularly detrimentally lead to overvoltages in conjunction with the e.g. parasitic inductances which are always present. This may cause oscillations in the drain-source voltage or the anode current, which impair the electromagnetic compatibility (EMC) of the switch or even destroy it in the most unfavorable case when the blocking ability is exceeded.
In order to resolve this problem, it has been proposed to reduced the charge carrier lifetime in the semiconductor switch by exposure to high-energy particle radiation (see, for example, “M. Schmitt, H.-J. Schulze, A. Schlögl, M. Vossebürger, A. Willmeroth, G. Deboy and G. Wachutka. A Comparison of Electron, Proton and Helium Ion Irradiation for the Optimization of the CoolMOS Body Diode, Proc. ISPSD, Santa Fe, 2002”). A reduction of the charge carrier lifetime can be carried out homogeneously over the component in this case, for example by irradiating it with electrons. The irradiation generates recombination centers in the component, which promote the recombination of charge carriers of opposite charge carrier types. The charge stored in the inverse diode can be reduced by the increased recombination rate, so that smaller switching losses are entailed and the reverse current peak can be significantly decreased in its magnitude. As a result, chopping of the reverse current leads to reduced-amplitude oscillations of the drain-source voltage, or the anode current.
A similar effect as by exposing the component to high-energy particle radiation can be achieved by diffusing metals, such as platinum or gold, into the component.
However, homogeneous charge carrier reductions can exert only little influence on the charge carrier distribution at the start of switching off. As shown in “J. Lutz, Freilaufdioden für schnell schaltende Anwendungen [freewheel diodes for fast-switching applications], dissertation TU Illmenau, Verlag ILSE, 2000”, to achieve a reverse current profile without chopping because of the different charge carrier mobilities, it is essential to produce a charge carrier distribution in the component which comprises a lower charge carrier density on the anode side than on the cathode side. To this end, either the doping of the channel zone (usually p-type doping) must be lowered in order to reduce the emitter efficiency of the anode of the diode, or the charge carrier lifetime must be locally reduced at this position, for example by helium irradiation. The former variant, however, always leads to an undesirable change in the properties of the MOSFET switch, while the latter variant entails significantly increased costs. Furthermore, in contrast to electron irradiation, the wafers cannot be stacked on one another for this process.
In another proposed solution, Schottky diodes are produced inside the semiconductor switch (see D. Calafut. Trench Power MOSFET Lowside Switch with Optimized Integrated Schottky Diode, Proc. ISPSD, Kitakyushu 2004). Such integration of Schottky diodes into the structure on the one hand allows very good properties of the inverse diode, but on the other hand also leads to a reduction of the active switch area and therefore to less effective utilization of the silicon surface. Furthermore, especially with high blocking voltages, care must be taken that the blocking current does not rise excessively.